This invention relates to radioactive well logging processes and systems for irradiating subterranean formations under investigation with bursts of fast neutrons and, more particularly, to an improved epithermal neutron detector for use in characterizing the formation on the basis of the decay of the subsequently produced epithermal neutron population.
Various techniques may be employed in order to characterize subterranean formations with regard to their fluid or mineral content, lithologic characteristics, porosity, or to provide for stratigraphic correlation. The neutron source may be a steady-state source or a pulsed source. For example, neutron porosity logging may be carried out using a steady-state neutron source in order to bombard the formation with fast neutrons. The porosity of the formation then may be determined by measuring thermal neutrons employing two detectors at different spacings from the source or by measuring epithermal neutrons with a single detector.
In pulsed neutron logging procedures, the formations are irradiated with repetitive bursts of fast neutrons, normally neutrons exhibiting an energy greater than 1 Mev. When the fast neutrons enter the formation, they are moderated, or slowed down, by nuclei within the formation to form lower energy neutron populations. The fast neutrons are moderated to lower energy levels by the nuclear collision processes of elastic and inelastic scattering. In elastic scattering the neutron loses a portion of its energy in a collision that is perfectly elastic, i.e., the energy lost by the neutron is acquired as kinetic energy by the nucleus with which it collides. In inelastic scattering only some of the energy lost by the neutrons is acquired as kinetic energy by the nucleus with which it collides. The remaining energy loss generally takes the form of a gamma ray emitted from the collision nucleus.
In the course of moderation, the neutrons reach the epithermal range and thence are further moderated until they reach the thermal neutron range. Thermal neutrons are neutrons which are in thermal equilibrium with their environment. The distribution in speed of thermal neutrons follows the so-called Maxwellian distribution law. The energy corresponding to the most probable speed for a temperature of 20.degree. C. is 0.025 electron volt. Epithermal neutrons are those neutrons which exhibit energies within the range from immediately above the thermal neutron region to about 100 electron volts. While the boundary between thermal and epithermal neutrons is, of necessity, somewhat arbitrary, it is normally placed in the range of 0.1-1.0 electron volt.
The populations of neutrons at the various energy levels decay with time following primary irradiation and thus offer means of characterizing the formation. For example, in the case of elastic scattering, which predominates for energies between a few electron volts and about 1 Mev, the number of collisions required for a neutron to moderate from one energy level to a second lower energy level varies more or less directly with the atomic weight of the nuclei available for collision. In subterranean formations, hydrogen nuclei present in hydrogenous materials such as oil, water, and gas tend to predominate in the slowing down process. Thus, the rate of decay of the epithermal neutron population gives a qualitative indication of the amount of hydrogenous material present which in turn may be indicative of the porosity of the formation.
For example, U.S. Pat. No. 4,097,737 to Mills discloses a method and system for epithermal neutron die-away logging utilizing a 14-Mev pulsed neutron source and a neutron detector that is sensitive to epithermal neutrons and highly discriminatory against thermal neutrons. The detector is relatively insensitive to the high energy neutrons and has a filter that renders it sharply insensitive to thermal neutrons.